Habitat destruction and fragmentation are the gravest threat to wildlife worldwide. We tested island biogeography theory, the correlation of habitat patch size to species richness and abundance of mammals and reptiles in a biodiversity hotspot California. Within nine forest fragments standardized for age, topography, climate, and vegetation cover we tested the effects of patch size and isolation on biodiversity. To measure species richness and abundance we used wildlife cameras for meso-and-large vertebrates, mark-and-recapture analyses for small mammals and reptiles, and standard dragging techniques for tick collections because they are frequent ecoparasites on vertebrates in oak woodland habitats. In addition to expected effects of habitat fragmentation on biodiversity, fragmentation may also influence infectious disease risk as an indirect consequence of changes in biodiversity, a concept known as the dilution-effect. In the western United States, the zoonotic pathogen causing Lyme disease, Borrelia burgdorferi (Bb), is transmitted between its vector, western blacklegged ticks (Ixodes pacificus), and vertebrate reservoir hosts. To measure Lyme disease risk along a fragmentation as well as a biodiversity gradient we collected ticks with standard dragging and flagging techniques and calculated the density of infected nymphs (DIN) a common measure for human Lyme disease risk.
Results/Conclusions
Our results show that meso-and-large vertebrate richness and abundance increases with patch area as does tick density. Surprisingly, small mammal species richness and abundance peaks in intermediate sized fragments. Resource limitation and competition at the smallest habitats and predation at the largest patches may be responsible for this pattern. Alternatively intermediate disturbance hypothesis might be applicable to the fragmentation context. Further there is a significant decrease in invasive species richness with habitat patch size. Tick density increases significantly with site area. It has been theorized that invasive species are better adapted than native species to thrive in the matrix surrounding natural habitat. Our results support this theory. We found that habitat destruction and fragmentation are acting upon species and communities in context-dependent ways that is critical to conservation planning, land-use design, ecosystem health, and ecosystem services.